Chromium exists primarily in trivalent (Cr(III)) or hexavalent (chromium(VI), Cr(VI) or Cr+6) oxidation states. Cr(VI) is a notorious environmental pollutant because it is strongly carcinogenic due to the ability of Cr(VI) ion in its water soluble form, CrO4=, to migrate in the biological fluids and to access sensitive living biological tissues, causing oxidative damage to these tissues. Cr(III) is reactively inert and relatively nontoxic; further, it is an essential trace nutrient in the diet of humans. All forms of hexavalent chromium are recognized by the United States National Institute of Environmental Health Sciences as a Group I known human carcinogen. Thus, hexavalent chromium is coming under increasing federal and state environmental regulations.
For metal surfaces, protective coatings containing hexavalent chromium (“chromate,” “Cr(VI),” or “Cr”) have been heavily used for many decades because of their excellent long term corrosion inhibition, enhanced coating adhesion, and self-sealing attributes, especially for coated and/or uncoated aluminum, steel, and zinc (galvanized) materials. During the process of applying these protective coatings, including re-finishing or stripping such coatings, the Cr(VI) materials enter the environment as dusts (airborne Cr(VI)) in places such as surface finishing shops, nondestructive inspection facilities, repair facilities and repainting areas.
To protect against airborne exposure to Cr(VI), the operators have to use paint mixing pots, paint spray guns, paint transfer hoses, air filters, rags and protective personal protection (i.e., Tyvek® suits, gloves, air-assisted face masks, and paper boots). The equipment so used becomes contaminated with the Cr(VI) dusts. These chromate containing dusts are very hazardous to human health. As such, agencies such as the U.S. Occupational Safety and Health Administration (OSHA) have strict guidelines for the levels of chromate allowed in the working environment. Current tough constraints on chromate contamination (Permissible Exposure Level or PEL is 5 μg Cr(VI)/m3 per 8-hour time worked average) by OSHA result in frequent citations to these refinishing shops for chromate contaminated dust violations.